Fix remap_path to only replace at the start of a string.

Fixes #6778.

Closes #6779.

PiperOrigin-RevId: 223043156
diff --git a/tools/build_defs/pkg/pkg.bzl b/tools/build_defs/pkg/pkg.bzl
index 93ff5da..7c98838 100644
--- a/tools/build_defs/pkg/pkg.bzl
+++ b/tools/build_defs/pkg/pkg.bzl
@@ -23,7 +23,7 @@
     """If path starts with a key in remap_paths, rewrite it."""
     for prefix, replacement in remap_paths.items():
         if path.startswith(prefix):
-            return path.replace(prefix, replacement)
+            return replacement + path[len(prefix):]
     return path
 
 def _quote(filename, protect = "="):